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LT3760
APPLICATIONS INFORMATION
V SUPPLY OFF = 1.476 ? 1 +
?
R2 ?
thesoftstartofV C pintobegin.Thisfeatureensuresthat
during startup of the LT3760 the soft start ramp has not
timed out before PWM is asserted high. Without this ‘wait
for PWM high’ feature, systems which apply PWM after V IN
and SHDN /UVLO are valid, can potentially turn on without
soft start and experience high inductor currents during
wake up of the converter’s output voltage. It is important
to note that when PWM subsequently goes low, the soft
start ramp is not held at its present voltage but continues
to ramp upwards. If the soft start ramp voltage was held
every time PWM goes low, this would cause very slow
startup of LED displays for applications using very high
PWM Dimming ratios.
Shutdown and Programming Undervoltage Lockout
The LT3760 has an accurate 1.476V shutdown threshold
at the SHDN /UVLO pin. This threshold can be used in
conjunction with a resistor divider from the system input
supply to define an accurate undervoltage lockout (UVLO)
threshold for the system (Figure 3). An internal hysteresis
current at the SHDN /UVLO pin allows programming of
hysteresis voltage for this UVLO threshold. Just before
part turn on, an internal 2.4μA flows from the SHDN /
V SUPPLY
UVLO pin. After part turn on, 0μA flows from the SHDN /
UVLO pin. Calculation of the turn on/off thresholds for a
system input supply using the LT3760 SHDN /UVLO pin
can be made as follows :
? R1 ?
?
V SUPPLY ON = V SUPPLY OFF + ( 2.4μA ? R1 )
An open drain transistor can be added to the resistor
divider network at the SHDN /UVLO pin to independently
control the turn off of the LT3760.
Programming Switching Frequency
The switching frequency of the LT3760 boost converter
can be programmed between 100kHz and 1MHz using a
single resistor (R T ) connected from the RT pin to ground
(Figure 4). Connect the R T resistor as close as possible to
the RT pin to minimize noise pick up and stray capacitance
(see “Circuit Layout Considerations” in the Applications
Information section). Table 5 shows the typical R T values
required for a range of frequencies.
1000
900
800
R1
15 SHDN /UVLO
–
700
600
OFF ON
R2
600k
1.476V
+
500
400
300
200
100
0
100
200
300
400
500
600
RT (k )
Figure 3. Programming Undervoltage
Lockout (UVLO) with Hysteresis
3760 F03
3760 F04
Figure 4. Switching Frequency vs RT
3760fc
15
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